The present invention relates to the removal of substances from sediments or mine tailings which are exposed to water. More specifically, it relates to the in situ restoration of contaminated lake, river or ocean sediments, to underwater mining and to the recovery of minerals and/or pollutants from mine tailings.
It is now generally recognized that in situ techniques for the restoration of contaminated lake and river sediments are less environmentally destructive than methods which require the dredging of the sediments. According to previous methods for in situ restoration of such sediments, media are added to lake sediments for the purpose of binding up the target pollutants and thereby reducing their impact on the overall chemical composition of the sediment. In the case of eutrophying lakes where excessive amounts of phosphate are present in the sediments, coagulants such as alum are added to bind the phosphate ions in insoluble metal salts. Similarly, it has been suggested that scrap iron added to lake sediments would bind mercury and thus decrease the potential for methylation.
Even though such binding procedures reduce the impact of the target pollutants they are not entirely satisfactory because they do not eliminate the contaminents from the water bodies. Since neither the pollutants nor the binding media are removed from the sediments, total pollutant content continues to increase even though a portion of pollutant may be bound up. Changes in water quality or the surrounding chemical environment can lead to the re-release of the bound up pollutant thus returning the water body to its original contaminated condition or worse. Also, the binding compounds become a permanent part of the water chemistry and thus alter the environment by their presence.
Underwater mining procedures are often similar to lake restoration procedures and subject to similar environmental restrictions. The dredging of sediments from river bottoms and even from sites on the ocean floor is a controversial and sometimes prohibited procedure. Environmentally safe methods for the recovery of metals and other substances from these sediments are widely sought after; but as yet few have been found.
Large piles of mine tailings are present in numerous locations throughout the world. In many instances these are exposed to the elements and thus subject to repeated washings with rain water. Many tailings contain water-soluble substances which are environmentally hazardous. Rainwater run-off from such tailings thus may transport the hazardous substances great distances from the location of the tailing pile, and may also disperse valuable water-soluble minerals which could have been recovered from the tailings. Heretofore no effective method of recovering water-soluble substances from the rainwater run-off of a mine tailings pile has been disclosed.